Preparation of 4,4'-dihydroxy-alpha-methylstilbene is delineated in an article entitled "Reactions of alpha-Halogeno-ketones with Aromatic Compounds. Part I. Reactions of Chloroacetone and 3-Chlorobutanone with Phenol and its Ethers" by S. H. Zaheer, et al., Journal of the Chemical Society, Part 3, pages 3360-3362 (1954). In a typical synthesis, concentrated sulfuric acid was added to a mixture of phenol and chloroacetone in a 2:1 mole ratio followed by workup to provide 4,4'-dihydroxy-alpha-methylstilbene. In a similar synthesis, substitution of 3-chlorobutan-2-one for the chloroacetone provided 2,3-di-p-hydroxyphenylbut-2-ene. The 4,4'-dihydroxy-alpha-methylstilbene produced via the method of Zaheer, et al. as obtained from workup was very impure, as was shown by the low melting point of 115.degree.-120.degree. C. A pair of recrystallizations from alcohol were required to raise the melting point to 176.degree.-179.degree. C., and even after a third recrystallization from benzene, the melting point was only 182.degree.-183.degree. C.
Preparation of 4,4'-dihydroxy-alpha-methylstilbene is delineated in an article entitled "Synthesis and Characterization of Thermotropic Polyethers and Copolyethers Based on 4,4'-Dihydroxy-alpha-methylstilbene and Flexible Spacers Containing Odd Numbers of Methylene Units" by V. Percec, et al., Mol. Cryst. Liq. Cryst., volume 205, pages 47-66 (1991). In the synthesis, concentrated sulfuric acid was added to a mixture of phenol and chloroacetone at -10 to -15 deg. C. to provide a 2.0:0.995:0.47 mole ratio of phenol:chloroacetone:sulfuric acid. Workup of the precipitated product required six recrystallizations from ethanol/water (6/4 vol./vol.) and provided a 5.9 percent yield of 4,4'-dihydroxy-alpha-methylstilbene with a purity of 99.6-99.9 percent by to which formalin solution was added. A residue was obtained upon workup which provided bis(4-hydroxyphenyl)methane upon dissolution in hot aqueous acetic acid followed by cooling. The mole ratio of phenol to formaldehyde used was 5.9:1.
All of the reactions of phenol and chloroacetone with an acid reported in the Zaheer, et al. article were performed at a reaction scale well below one mole of phenol. In the Percec, et al. article, the reaction was performed using a mole ratio of phenol:chloroacetone:sulfuric acid of 1.26:0.627:0.296, which is less sulfuric acid stoichiometry than needed to fully react all of the phenol and chloroacetone present. In the hands of the present inventors, attempts to use the reaction methods of Zaheer, et al. at their stoichiometric ratios of 2:1:1 phenol:chloroacetone:sulfuric acid at a reaction scale using one or greater moles of phenol invariably induced the formation of a highly viscous reaction mixture at the latter stages of the reaction followed by an exothermic decomposition of the reaction product. Thus as an improvement upon the method of Zaheer, et al., the present inventors discovered that a solvent, e.g. methylene chloride, could be used to moderate reaction viscosity and thus allow for heat transfer required for scaleups based on a reaction scale of one or more moles of phenol. This use of the solvent is indicated in the Comparative Experiments provided herein. Also, the present inventors evaluated, as indicated in the Comparative Experiments provided herein, the typically aforementioned use of a substantial excess of phenol to ketone stoichiometry in bisphenol synthesis to attempt to minimize coproduct formation and provide a product with reasonable purity.
While the use of a solvent and the adaptation of the excess phenol to chloroacetone stoichiometry provided the present inventors with 4,4'-dihydroxy-alpha-methylstilbene of acceptable purity for numerous end uses such as, for example, preparation of epoxy resins, the need exists for 4,4'-dihydroxy-alpha-methylstilbene of still higher purity for such uses as linear advancement of epoxy resins. It was recognized that purity improvements meant not only reduction of coproduct levels obtained in the current process, but also elimination of coproducts wherever possible. Furthermore, the need exists to simplify the process for producing 4,4'-dihydroxy-alpha-methylstilbene by eliminating the need for an additional chemical, the solvent such as methylene chloride, used in the process and its subsequent removal. Additionally, obtaining isolated 4,4'-dihydroxy-alpha-methylstilbene yields above the relatively modest yields provided by the current processing methods was deemed to be desirable.
In the present invention it was surprisingly found that the use of alpha-haloketone in the synthesis of 4,4'-dihydroxy-alpha-alkylstilbenes using phenol or alkoxybenzene:alpha-haloketone mole ratios of less than 2:1 provides one or more of the aforementioned improvements such as increased product purity, elimination of the need for solvent, or increase in isolated product yield.